1. Field of the Invention
This invention relates generally to downhole tools, and more particularly to downhole logging tools capable of casing inspection.
2. Description of the Related Art
A well casing is a metal pipe inserted into a borehole to provide mechanical support for the borehole and to enable the driller to control the types of subsurface fluids allowed to enter the borehole and the locations for such entries. Well casings are normally constructed of ferromagnetic steels. As such, they are subject to corrosion, mechanical damage due to impacts from downhole tools and even warpage resulting from stresses imparted by shifting borehole formations. Accordingly, determining the condition of well casings through inspection is an important part of well drilling and management.
Casing inspection based on the remote-field eddy current principle has been widely used for casing inspection in oil and gas industry for decades. Conventional techniques use a transmitter to generate a magnetic field and a receiver to sense the magnetic field and then calculate the average casing thickness at a given depth based on the phase shift and amplitude attenuation of the magnetic field. In remote field eddy current techniques the receiver is placed in the remote-field zone, which is displaced vertically from the transmitter a distance greater than twice the casing inner diameter. The magnetic field created by a magnetic dipole source located inside the casing propagates in three zones: the direct coupling zone, the transition zone and the remote field zone. When the transmitter is fired, typically below 100 Hz, and generates direct field inside the casing, an eddy current is generated on the casing wall and forms a field against the primary field from the transmitter. Inside the casing and away from the transmitter, the direct field from the transmitter decays exponentially and rapidly due to eddy current loops. However, the magnetic field also penetrates through the casing wall and propagates outside the casing, where it encounters the formation and perhaps an annulus filled with cement. This component of the propagating magnetic field that penetrates through the casing wall and propagates outside the casing is the remote field. After penetrating the casing wall, the remote field attenuates due primarily to the media outside the casing, e.g., the cement and formation. So if the region outside the casing consists of low conductive media, the remote field decays much more slowly than the direct field. The field inside the casing is also affected by the remote field since the remote field always propagates back through the casing wall where it will undergo another decay. The final field inside the casing is thus made up of a superposition of the direct field and the remote field. Therefore, there is a direct coupling zone near the transmitter where the direct field dominates, the remote field zone where the field propagating back from outside the casing is much stronger and dominates, and the transition zone between the two where neither the direct field nor the remote field predominates, so neither can be ignored. Phase shift and amplitude attenuation measurements are made in the remote field zone. The measured relative signal phase shift is proportional to the casing wall thickness, casing conductivity and permeability. Once the other two parameters are pre-determined, the casing wall thickness can be accurately derived.
Manufacturers have produced various conventional casing inspection tool designs over the years. The Schlumberger Multi-Frequency Electromagnetic Thickness Tool (METT) utilized multiple coils and frequencies to solve the casing properties and the thickness at the same time in order to achieve better thickness accuracy. Other conventional remote-field eddy current tools include the Digital Magnelog (DMAG) from Baker Hughes, the Multi-Frequency Electromagnetic Thickness Gauge (METG) and Casing Inspection Tool (CIT) from Halliburton, Omni-Directional Thickness (ODT) from Hotwell, the Induction Collar Locator (ICL) from CBG Corp. and the Electromagnetic Inspection Tool (EMIT) from Probe Technology Services, Inc., the assignee of the present application. The measurements from all the technologies mentioned above are uni-directional and only indicate the circumferential average of the casing wall thickness.
There have been a few conventional designs that target an azimuthal casing thickness measurement. One variant is the Magnetic Thickness Tool (MTT) from GE Sondex. This design uses multiple sensors, mainly receivers, positioned on bow-springs or pads, which are usually extended out from the tool mandrel. The EM Pipe Scanner from Schlumberger, is another variant in this category. There are several issues associated with the bow-spring/pad mounted approach. First, the mechanical complexity of these designs introduces significant manufacturing and maintenance costs. Second, operational reliability may be problematic because of the moving parts.
Finally, one other conventional approach for azimuthal casing thickness measurement involves taking a measurement with a sensor, then physically rotating the sensor portion of the tool by some motorized mechanism. Again, cost and mechanical reliability remain issues with this technique.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.